U.S. patent application number 15/549252 was filed with the patent office on 2018-02-08 for a quantum dot apparatus and associated methods and apparatus.
The applicant listed for this patent is EMBERION OY. Invention is credited to Chris BOWER, Elisabetta SPIGONE.
Application Number | 20180040750 15/549252 |
Document ID | / |
Family ID | 52462197 |
Filed Date | 2018-02-08 |
United States Patent
Application |
20180040750 |
Kind Code |
A1 |
BOWER; Chris ; et
al. |
February 8, 2018 |
A QUANTUM DOT APPARATUS AND ASSOCIATED METHODS AND APPARATUS
Abstract
A method comprising: depositing a quantum dot solution onto a
supporting substrate in a drop-wise manner to form one or more
discrete droplets on a surface of the substrate, the quantum dot
solution and discrete droplets comprising a plurality of quantum
dots having primary ligands attached thereto to stabilise the
quantum dots in solution; and depositing a ligand-exchange solution
onto the one or more discrete droplets in a drop-wise manner to
cause replacement of the primary ligands attached to the plurality
of quantum dots with shorter-chain secondary ligands, replacement
of the primary ligands with the secondary ligands allowing the
plurality of quantum dots within each discrete droplet to become
sufficiently close packed to facilitate charge transfer there
between.
Inventors: |
BOWER; Chris; (Ely, GB)
; SPIGONE; Elisabetta; (Cambridge, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EMBERION OY |
Espoo |
|
FI |
|
|
Family ID: |
52462197 |
Appl. No.: |
15/549252 |
Filed: |
January 28, 2016 |
PCT Filed: |
January 28, 2016 |
PCT NO: |
PCT/FI2016/050044 |
371 Date: |
August 7, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/035218 20130101;
B82Y 15/00 20130101; B82Y 20/00 20130101; H01L 51/0015 20130101;
C09D 11/322 20130101; B41J 2/2114 20130101; C09D 11/52 20130101;
H01L 51/0005 20130101 |
International
Class: |
H01L 31/0352 20060101
H01L031/0352; C09D 11/52 20060101 C09D011/52; B82Y 15/00 20060101
B82Y015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2015 |
EP |
15154230.5 |
Claims
1. A method, comprising: depositing a quantum dot solution onto a
supporting substrate in a drop-wise manner to form one or more
discrete droplets on a surface of the substrate, the quantum dot
solution and discrete droplets comprising a plurality of quantum
dots having primary ligands attached thereto to stabilise the
quantum dots in solution; and depositing a ligand-exchange solution
onto the one or more discrete droplets in a drop-wise manner to
cause replacement of the primary ligands attached to the plurality
of quantum dots with shorter-chain secondary ligands, replacement
of the primary ligands with the secondary ligands allowing the
plurality of quantum dots within each discrete droplet to become
sufficiently close packed to facilitate charge transfer
therebetween.
2. The method of claim 1, wherein the method comprises depositing a
rinse solution onto the one or more droplets in a drop-wise manner
to remove the primary ligands and any excess secondary ligands from
the substrate.
3. The method of claim 2, wherein the method comprises evaporating
the rinse solution after deposition to aid removal of the primary
and excess secondary ligands from the substrate.
4. The method of claim 3, wherein the method comprises controlling
one or more of the temperature of the substrate, the surface
chemistry of the substrate, and the composition of the rinse
solution to facilitate evaporation of the rinse solution.
5. The method of claim 1, wherein the method comprises: allowing
the one or more discrete droplets to dry after deposition of the
quantum dot and ligand-exchange solutions to form two or more
corresponding quantum dot regions on the surface of the substrate;
and repeating the deposition and drying steps until the two or more
quantum dot regions have a predetermined thickness.
6. The method of claim 1, wherein the quantum dot solution and
discrete droplets further comprise an electrically conductive
material, the quantum dots and electrically conductive material
arranged within the quantum dot solution and discrete droplets to
form a composite material.
7. The method of claim 1, wherein the method comprises depositing a
functional material onto the one or more discrete droplets to one
or more of functionalise the plurality of quantum dots within the
droplets for a particular application and encapsulate the quantum
dots to provide environmental stability.
8. The method of claim 1, wherein the quantum dot solution is
deposited to form a one, two or three dimensional array of discrete
droplets on the surface of the substrate.
9. An apparatus, comprising: one or more discrete quantum dot
regions on a surface of a supporting substrate, wherein each
quantum dot region comprises a plurality of quantum dots separated
from one another by secondary ligands attached thereto, and wherein
the secondary ligands have a chain length which is sufficiently
short to facilitate charge transfer between the plurality of
quantum dots within the quantum dot region.
10. A deposition tool, comprising: first and second deposition
heads each having a respective cartridge, wherein the first
deposition head is configured to deposit a quantum dot solution
from its respective cartridge in a drop-wise manner, the quantum
dot solution comprising a plurality of quantum dots having primary
ligands attached thereto to stabilise the quantum dots in solution,
and wherein the second deposition head is configured to deposit a
ligand-exchange solution from its respective cartridge in a
drop-wise manner, the ligand-exchange solution configured to enable
replacement of the primary ligands attached to the plurality of
quantum dots with shorter-chain secondary ligands.
11. The deposition tool of claim 10, wherein the first and second
deposition heads are configured to be moved independently of one
another.
12. The deposition tool of claim 10, wherein the first and second
deposition heads form constituent parts of a combined deposition
head so as to be configured to be moved together.
13. The deposition tool of claim 10, wherein the deposition tool
comprises a third deposition head having a respective cartridge,
the third deposition head configured to deposit a rinse solution
from its respective cartridge in a drop-wise manner, the rinse
solution configured to enable removal of the primary ligands and
any excess secondary ligands.
14. The deposition tool of claim 10, wherein the deposition tool
comprises a plurality of the first and second deposition heads
together with their respective cartridges.
15. A computer program embodied on a non-transitory
computer-readable medium, said computer program comprising computer
code configured to control a processor to perform the method of
claim 1.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to the field of quantum dots
(where a quantum dot encompasses a nanocrystal with optical
properties resulting as a consequence of the finite size effects
that occur in collections of a few 100 atoms or less, where the
morphology of the nanocrystal can be spherical, rod-like,
plate-like etc.), associated methods and apparatus, and in
particular concerns a method of forming a quantum dot apparatus in
which quantum dot and ligand-exchange solutions are deposited in a
drop-wise manner to form one or more discrete droplets on the
surface of a supporting substrate. Certain disclosed example
aspects/embodiments relate to portable electronic devices, in
particular, so-called hand-portable electronic devices which may be
hand-held in use (although they may be placed in a cradle in use).
Such hand-portable electronic devices include so-called Personal
Digital Assistants (PDAs) and tablet PCs.
[0002] The portable electronic devices/apparatus according to one
or more disclosed example aspects/embodiments may provide one or
more audio/text/video communication functions (e.g.
tele-communication, video-communication, and/or text transmission,
Short Message Service (SMS)/Multimedia Message Service
(MMS)/emailing functions, interactive/non-interactive viewing
functions (e.g. web-browsing, navigation, TV/program viewing
functions), music recording/playing functions (e.g. MP3 or other
format and/or (FM/AM) radio broadcast recording/playing),
downloading/sending of data functions, image capture function (e.g.
using a (e.g. in-built) digital camera), and gaming functions.
BACKGROUND
[0003] Research is currently being done to develop new optical
sensors, some of which comprise quantum dots for greater design
flexibility and improved optical efficiency.
[0004] The listing or discussion of a prior-published document or
any background in this specification should not necessarily be
taken as an acknowledgement that the document or background is part
of the state of the art or is common general knowledge.
SUMMARY
[0005] According to a first aspect, there is provided a method
comprising: [0006] depositing a quantum dot solution onto a
supporting substrate in a drop-wise manner to form one or more
discrete droplets on a surface of the substrate, the quantum dot
solution and discrete droplets comprising a plurality of quantum
dots having primary ligands attached thereto to stabilise the
quantum dots in solution; and [0007] depositing a ligand-exchange
solution onto the one or more discrete droplets in a drop-wise
manner to cause replacement of the primary ligands attached to the
plurality of quantum dots with shorter-chain secondary ligands,
replacement of the primary ligands with the secondary ligands
allowing the plurality of quantum dots within each discrete droplet
to become sufficiently close packed to facilitate charge transfer
there between.
[0008] The method may comprise: [0009] depositing a quantum dot
solution onto a supporting substrate in a drop-wise manner to form
two or more discrete droplets on a surface of the substrate, the
quantum dot solution and discrete droplets comprising a plurality
of quantum dots having primary ligands attached thereto to
stabilise the quantum dots in solution; and [0010] depositing a
ligand-exchange solution onto the two or more discrete droplets in
a drop-wise manner to cause replacement of the primary ligands
attached to the plurality of quantum dots with shorter-chain
secondary ligands, replacement of the primary ligands with the
secondary ligands allowing the plurality of quantum dots within
each discrete droplet to become sufficiently close packed to
facilitate charge transfer there between.
[0011] The term quantum dot encompasses a nanocrystal with optical
properties resulting as a consequence of the finite size effects
that occur in collections of a few 100 atoms or less, where the
morphology of the nanocrystal can be spherical, rod-like,
plate-like or other shape.
[0012] The quantum dot solution may be deposited such that the one
or more discrete droplets have a diameter of no more than 5 .mu.m,
10 .mu.m, 25 .mu.m, 50 .mu.m or 100 .mu.m. The diameter may range
from a few microns to hundreds of microns.
[0013] The quantum dot solution may be deposited such that the one
or more discrete droplets are spaced apart by no more than 1 .mu.m,
5 .mu.m, 10 .mu.m or 15 .mu.m. Typically, the spacing may be a few
microns (up to around 10 .mu.m).
[0014] The quantum dots may be spaced apart from one another by at
least 1.5 nm, 2 nm, 2.5 nm or 5 nm before ligand replacement.
[0015] The quantum dots may be spaced apart from one another by no
more than 0.25 nm, 0.5 nm, 0.75 nm, 1 nm or 1.25 nm after ligand
replacement or removal.
[0016] The quantum dot solution may comprise organic aromatic
solvents such as toluene, chlorobenzene, xylene, NMP or organic no
aromatic solvents such as hexane, octane or other alkane. Aqueous
solvents are also possible, the quantum dots may comprise PbS,
PbSe, PbTe, CdS, CdSe, CdTe, InP, InAs, HgTe, ZnSe, CdSeTe, CdHgTe,
ZnSeS, ZnHgSe, ZnSeTe or combinations of these materials.
[0017] The ligand-exchange solution may comprise acetonitrile.
[0018] The primary ligands may comprise one or more of thio-,
amino-, carboxylic-, phosphonato-, sulfonate or alkyl or aromatic
tail groups such as: oleic acid, oleate, trioctylphosphine oxide,
alkylphosphonic acid, fatty acid and long-chain alkylamine.
[0019] The secondary ligands may comprise one or more of
1,2-ethanedithiol, methylamine, ethylamine, ethylene diamine,
ethanethiol, propanethiol, 1,3-benzenedithiol, hydrazine, formic
acid, oxalic acid, acetic acid, pyridine, butylamine, formamide,
SnS.sub.4, PbBr.sub.2, Pbl.sub.2 and PbCl.sub.2, Na2S.9H2O,
KOH.
[0020] The method may comprise depositing a rinse solution onto the
two or more droplets in a drop-wise manner to remove the primary
ligands and any excess secondary ligands from the substrate.
[0021] The rinse solution may comprise one or more of acetonitrile
and toluene.
[0022] Depositing one or more of the quantum dot, ligand-exchange
and rinse solutions in a drop-wise manner may comprise depositing
drops of no more than 1 ml, 1 .mu.l, 1 .mu.l or 1 fl in volume.
[0023] The method may comprise evaporating the rinse solution after
deposition to aid removal of the primary and excess secondary
ligands from the substrate.
[0024] The method may comprise controlling one or more of the
temperature of the substrate, the surface chemistry of the
substrate, and the composition of the rinse solution to facilitate
evaporation of the rinse solution.
[0025] The method may comprise: [0026] allowing the one or more
discrete droplets to dry after deposition of the quantum dot and
ligand-exchange solutions to form two or more corresponding quantum
dot regions on the surface of the substrate; and [0027] repeating
the deposition and drying steps until the one or more quantum dot
regions have a predetermined thickness.
[0028] The quantum dot solution and discrete droplets may further
comprise an electrically conductive material, the quantum dots and
electrically conductive material arranged within the quantum dot
solution and discrete droplets to form a composite material.
[0029] The method may comprise depositing a functional material
onto the one or more discrete droplets to functionalise the
plurality of quantum dots within the droplets for a particular
application or encapsulate the quantum dots to provide
environmental stability.
[0030] The functional material may comprise one or more of a
biological, metallic nanoparticle, semiconducting and
photosensitive material.
[0031] The functional material may comprise one or more of an
antibody, an antigen, a protein, a DNA fragment, a conjugated
polymer and a dye.
[0032] The quantum dot solution may be deposited to form a one, two
or three dimensional array of discrete droplets on the surface of
the substrate. Combinations of the same or different quantum dot
materials with the same size or different sizes can be printed in
alternate layers to build more complex architectures.
[0033] According to a further aspect, there is provided an
apparatus comprising one or more discrete quantum dot regions on a
surface of a supporting substrate, wherein each quantum dot region
comprises a plurality of quantum dots separated from one another by
secondary ligands attached thereto, and wherein the secondary
ligands have a chain length which is sufficiently short to
facilitate charge transfer between the plurality of quantum dots
within the quantum dot region.
[0034] The one or more discrete quantum dot regions may have a
thickness of at least 10 nm, 100 nm, 1 .mu.m, 10 .mu.m or 100
.mu.m.
[0035] The secondary ligands may have a chain length of no more
than 0.1 nm, 0.25 nm, 0.5 nm, 0.75 nm, 1 nm or 1.25 nm.
[0036] The substrate may comprise an electrically conductive
material configured to transfer charge from the quantum dots.
[0037] The substrate may be formed from or coated by the
electrically conductive material.
[0038] The electrically conductive material may comprise one or
more of carbon, graphene, reduced graphene oxide, carbon nanotubes,
a metal and a semiconductor.
[0039] The apparatus may comprise first and second electrodes
configured to enable a flow of electrical current from the first
electrode through the electrically conductive material to the
second electrode.
[0040] The plurality of quantum dots may comprise one or more of
CdSe, CdS, PbSe, PbS, ZnO, ZnS, CZTS, Cu.sub.2S, Bi.sub.2S.sub.3,
Ag.sub.2S, HgTe, CdSe, CdHgTe, InAs, InSb, PbTe, CdTe, InP, ZnSe,
CdSeTe, ZnSeS, ZnHgSe, ZnSeTe Ge and CIS.
[0041] The substrate may comprise one or more of diamond like
carbon (DLC), Si, GaAs, BN, SiO.sub.2, LiF, Al.sub.2O.sub.3,
HfO.sub.2 and graphene.
[0042] The apparatus may be one or more of an electronic device, a
portable electronic device, a portable telecommunications device, a
mobile phone, a personal digital assistant, a tablet, a phablet, a
desktop computer, a laptop computer, a server, a smartphone, a
smartwatch, smart eyewear, a sensor, a camera, an image sensor, a
photodetector, a phototransistor, and a module for one or more of
the same.
[0043] According to a further aspect, there is provided a
deposition tool comprising first and second deposition heads each
having a respective cartridge, [0044] wherein the first deposition
head is configured to deposit a quantum dot solution from its
respective cartridge in a drop-wise manner, the quantum dot
solution comprising a plurality of quantum dots having primary
ligands attached thereto to stabilise the quantum dots in solution,
and [0045] wherein the second deposition head is configured to
deposit a ligand-exchange solution from its respective cartridge in
a drop-wise manner, the ligand-exchange solution configured to
enable replacement of the primary ligands attached to the plurality
of quantum dots with shorter-chain secondary ligands.
[0046] The first and second deposition heads may be configured to
be moved independently of one another.
[0047] The first and second deposition heads may form constituent
parts of a combined deposition head so as to be configured to be
moved together.
[0048] The deposition tool may comprise a third deposition head
having a respective cartridge, the third deposition head configured
to deposit a rinse solution from its respective cartridge in a
drop-wise manner, the rinse solution configured to enable removal
of the primary ligands and any excess secondary ligands.
[0049] The deposition tool may comprise a plurality of the first
and second deposition heads together with their respective
cartridges.
[0050] The deposition tool may be one or more of an inkjet, spray,
electrospray and valve-jet system.
[0051] The steps of any method disclosed herein do not have to be
performed in the exact order disclosed, unless explicitly stated or
understood by the skilled person.
[0052] Corresponding computer programs (which may or may not be
recorded on a carrier) for implementing one or more of the methods
disclosed herein are also within the present disclosure and
encompassed by one or more of the described example
embodiments.
[0053] The present disclosure includes one or more corresponding
aspects, example embodiments or features in isolation or in various
combinations whether or not specifically stated (including claimed)
in that combination or in isolation. Corresponding means for
performing one or more of the discussed functions are also within
the present disclosure.
[0054] The above summary is intended to be merely exemplary and
non-limiting.
BRIEF DESCRIPTION OF THE FIGURES
[0055] A description is now given, by way of example only, with
reference to the accompanying drawings, in which:--
[0056] FIG. 1 illustrates schematically one example of a method of
forming a quantum dot apparatus;
[0057] FIG. 2 illustrates schematically one example of a deposition
tool for use in a method described herein;
[0058] FIG. 3a is an optical micrograph of a substrate comprising
two rows of droplets formed from 11 drops of quantum dot
solution;
[0059] FIG. 3b is an optical micrograph of the substrate of FIG. 3a
further comprising two rows of droplets formed from 6 drops of
quantum dot solution;
[0060] FIG. 3c is an optical micrograph of the substrate of FIG. 3b
further comprising two rows of droplets formed from 4 drops of
quantum dot solution;
[0061] FIG. 3d is an optical micrograph of the substrate of FIG. 3c
following deposition of a ligand-exchange solution onto three rows
of droplets;
[0062] FIG. 3e is an optical micrograph of the substrate of FIG. 3d
following deposition of a first rinse solution;
[0063] FIG. 3f is an optical micrograph of the substrate of FIG. 3e
following deposition of a second rinse solution;
[0064] FIG. 4a is an optical micrograph of the substrate of FIG. 3f
at a low level of magnification;
[0065] FIG. 4b is an optical micrograph of the substrate of FIG. 3f
at a medium level of magnification;
[0066] FIG. 4c is an optical micrograph of the substrate of FIG. 3f
at a high level of magnification;
[0067] FIG. 5a is an AFM image and corresponding height profile of
a quantum dot region of FIG. 4a with no ligand replacement;
[0068] FIG. 5b is an AFM image and corresponding height profile of
a quantum dot region of FIG. 4a after ligand replacement;
[0069] FIG. 6a is a conductive AFM image of the quantum dot region
of FIG. 5a;
[0070] FIG. 6b is a conductive AFM image of the quantum dot region
of FIG. 5b;
[0071] FIG. 7 illustrates schematically one example of a quantum
dot apparatus configured to detect incident electromagnetic
radiation;
[0072] FIG. 8 illustrates schematically another example of a
quantum dot apparatus;
[0073] FIG. 9 illustrates schematically the main steps of a method
described herein;
[0074] FIG. 10 illustrates schematically a computer-readable medium
comprising a computer program configured to perform, control or
enable one or more steps of a method described herein.
DESCRIPTION OF SPECIFIC ASPECTS/EMBODIMENTS
[0075] Sensors currently exist for detecting a wide variety of
different stimuli, including light, temperature and chemical or
biological species. Many sensors are based on thin-film transistors
which can be used as transducers to make sensitive measurements of
the physical parameter. In these devices, the stimuli being sensed
influence the flow of charge carriers through the transistor
channel from a source electrode to a drain electrode, and the level
of current flow provides an indication of the physical
parameter.
[0076] The devices can be made sensitive to a specific stimuli by
functionalization of the channel material, e.g. by using an
optically sensitive material which responds to a particular
wavelength of light, or a biological molecule which binds to a
particular analyte. Quantum dots are suitable for use as optically
sensitive materials because they have a narrow absorption
wavelength that can be controlled by changing the size of the
quantum dots, and exhibit a relatively high quantum efficiency.
Quantum dots can also be fabricated from low cost, solution
processed materials, can be manufactured at room temperature and
can be easily integrated with existing CMOS and semiconductor
devices, all of which facilitate large scale manufacture.
[0077] In device fabrication, quantum dots are typically deposited
from solution onto a substrate to form a close-packed layer. The
formation of a close-packed layer enables charge transfer between
the dots for use in detection of the incident electromagnetic
radiation. In order to passivate the reactive surface of the
quantum dots and reduce their aggregation within the solution, the
quantum dots must be stabilized with ligands. The ligands are
generally long-chain (.about.2 nm) fatty acids such as oleic acid
which make the quantum dots soluble in non-polar organic solvents
such as toluene. These long-chain ligands can hinder the formation
of a close-packed quantum dot layer and are therefore typically
replaced by shorter-chain ligands having a chain length which is
sufficiently short to enable charge transfer between the quantum
dots.
[0078] The fabrication process usually involves several
time-consuming processing steps. Once such method involves
spin-coating the substrate sequentially with quantum dot,
ligand-exchange and rinse solutions. A disadvantage of this
technique, however, is that it is difficult to get the
ligand-exchange solution to infiltrate and perform ligand
replacement on quantum dots that are deep within the layer. This
becomes even more of an issue for thicker layers of quantum dots
(e.g. >100 nm), which are often desirable in applications such
as X-ray detection to improve absorption of the incident
electromagnetic radiation.
[0079] Current fabrication methods are therefore time consuming and
limit the thickness of the quantum dot layers that can be
deposited.
[0080] There will now be described an apparatus and associated
methods that may or may not provide a solution to one or more of
these issues.
[0081] FIG. 1 illustrates schematically a method of forming a layer
of quantum dots suitable for use in a sensor. The method comprises
depositing (e.g. via a first deposition head 102 of a deposition
tool) a quantum dot solution 103 onto a supporting substrate 101 in
a drop-wise manner to form two or more discrete droplets on a
surface of the substrate 101, the quantum dot solution 103 and
discrete droplets comprising a plurality of quantum dots having
primary ligands attached thereto to stabilise the quantum dots in
solution. After the one or more discrete droplets of quantum dot
solution 103 have been deposited, a ligand-exchange solution 105 is
then deposited (e.g. via a second deposition head 104 of the
deposition tool) onto the discrete droplets (before or after the
solvent of the quantum dot solution 103 has evaporated, depending
on the particular solutions 103, 105) in a drop-wise manner to
cause replacement of the primary ligands attached to the plurality
of quantum dots with shorter-chain secondary ligands. As mentioned
previously, replacement of the primary ligands with the secondary
ligands allows the plurality of quantum dots within each discrete
droplet to become sufficiently close packed to facilitate charge
transfer there between.
[0082] After ligand replacement has occurred, a rinse solution 107
may optionally be deposited onto the two or more droplets (before
or after the solvent of the ligand-exchange solution 105 has
evaporated, depending on the particular solutions 105, 107) to
remove the primary ligands and any excess secondary ligands from
the substrate 101. Removal of the primary ligands from the
substrate 101 helps to prevent their reattachment to the quantum
dots. As shown in FIG. 1, the rinse solution 107 may be deposited
via a third deposition head 106 of the deposition tool in a
drop-wise manner similar to the quantum dot 103 and ligand-exchange
105 solutions. Alternatively, the rinse solution 107 could be
deposited globally by washing the substrate surface with the rinse
solution 107. Once the rinse solution 107 has been removed from the
substrate 101 (together with the primary and any excess secondary
ligands), the substrate surface comprises two or more discrete
quantum dot regions 108 each comprising a plurality of quantum dots
which are separated from one another by secondary ligands. These
secondary ligands have a chain length which is sufficiently short
(e.g. no more than 0.1 nm, 0.25 nm, 0.5 nm, 0.75 nm, 1 nm or 1.25
nm) to enable charge transfer between the plurality of quantum dots
within the quantum dot region 108.
[0083] As mentioned above, certain applications require quantum
dots layers with a thickness of over 100 nm (e.g. at least 1 .mu.m,
10 .mu.m or 100 .mu.m). This can achieved with the present method
simply by allowing the two or more discrete droplets to dry after
deposition of the various solutions 103, 105, 107, and repeating
the deposition and drying steps until the resulting quantum dot
regions 108 have a predetermined thickness. Since ligand
replacement is performed on each individual droplet of quantum dot
solution 103, it is possible to build up thicker layers of
close-packed quantum dots than is currently possible.
[0084] Furthermore, by depositing discrete droplets of quantum dot
103, ligand-exchange 105 and rinse 107 solutions in a drop-wise
manner (e.g. drops of no more than 1 ml, 1 .mu.l, 1 pl or 1 fl in
volume) the solvents within these solutions 103, 105, 107 are able
to evaporate relatively quickly. This feature enables the use of
solutions 103, 105, 107 having boiling points of over 100.degree.
C. without long waiting times between each step in situations where
it is desirable to evaporate the solvent before the subsequent
deposition. In practice, a number of different solvents can be used
to form the quantum dot 103, ligand-exchange 105 and rinse 107
solutions. Nevertheless, the particular solvent used in the quantum
dot solution 103 will typically depend on the materials used to
form the quantum dots and primary ligands, and the particular
solvents used in the ligand-exchange 105 and rinse 107 solutions
will typically depend on the materials used to form the primary and
secondary ligands. The type of deposition tool used to deposit the
various solutions 103, 105, 107 may also affect the choice of
solvent. As one particular example, the quantum dot solution 103
may comprise PbS quantum dots having oleic acid primary ligands
within a toluene solvent, the ligand-exchange solution 105 may
comprise 1,2-ethanedithiol secondary ligands within an acetonitrile
solvent, and the rinse solution 107 may comprise one or more of
toluene and acetonitrile.
[0085] The quantum dots, primary ligands and secondary ligands are
not limited (respectively) to PbS, oleic acid and
1,2-ethanedithiol. The quantum dots may be formed from one or more
of CdSe, CdS, PbSe, PbS, ZnO, ZnS, CZTS, Cu.sub.2S,
Bi.sub.2S.sub.3, Ag.sub.2S, HgTe, CdSe, CdHgTe, InAs, InSb, Ge and
CIS. Similarly, other examples of relatively long-chain primary
ligands include oleate, trioctylphosphine oxide, alkylphosphonic
acid, fatty acid or long-chain alkylamine; and other examples of
relatively short-chain secondary ligands include ethylene diamine,
ethanethiol, propanethiol, 1,3-benzenedithiol, hydrazine, formic
acid, oxalic acid, acetic acid, pyridine, butylamine, SnS.sub.4,
PbBr.sub.2, Pbl.sub.2 and PbCl.sub.2.
[0086] In some cases, it may be desirable to evaporate the rinse
solution 107 after deposition to aid removal of the primary and
excess secondary ligands from the substrate 101. A number of
different techniques can be used to facilitate evaporation of the
rinse solution 107, such as controlling one or more of the
temperature of the substrate 101, the surface chemistry of the
substrate 101, and the composition of the rinse solution 107. These
techniques can be used to manipulate the Marangoni flows within the
drying droplets to change the drying behaviour of the droplets.
Under certain conditions, there may be fluid flow towards the edges
of the droplet which drives increased evaporation at the droplet
edges. Under different conditions, the evaporation may occur more
uniformly over the entire surface of the droplet.
[0087] As illustrated in FIG. 1, the deposition tool used to
perform the method comprises first 102, second 104 and possibly
third 106 deposition heads, each having a respective cartridge
which may or may not form part of the deposition head 102, 104, 106
itself. The deposition heads 102, 104, 106 are configured to
deposit the respective solution 103, 105, 107 from their respective
cartridge in a drop-wise manner, each deposition head comprising a
nozzle 110 from which the respective solution 103, 105, 107 is
ejected (although in other embodiments, the respective cartridges
may each have a respective exit therefrom but share a common
ejection nozzle 110). In the example shown in FIG. 1, the first
102, second 104 and third 106 deposition heads are configured to be
moved independently of one another. This arrangement provides
flexibility in the time interval between depositions.
[0088] FIG. 2 shows another example of a deposition tool for use in
the method described herein. In this example, the first 202, second
204 and third 206 deposition heads form constituent parts of a
combined deposition head 209 so as to be configured to be moved
together. Furthermore, the deposition tool can be configured such
that the distance moved by the combined deposition head 209 between
subsequent depositions is substantially the same as the distance
between the nozzles of the constituent parts 202, 204, 206. This
feature helps to ensure that the ligand-exchange 205 and rinse 207
solutions are deposited onto each droplet of quantum drop solution
203. The arrangement of FIG. 2 reduces the overall deposition time
since all the printheads are co-located allowing for a single-pass
deposition. In some scenarios, the constituent parts 202, 204, 206
of the combined deposition head 209 may have independent control
over the volume of the drops deposited and independent control over
the timings of the depositions and the deposition of QDs, ligand
replacement and rinse solutions may be done in a single printing
pass or in multiple printing passes.
[0089] In both configurations of deposition tool, there may be a
plurality of first 202, second 204 and/or third 206 deposition
heads together with their respective cartridges. The use of
multiple heads 202, 204, 206 for each solution 203, 205, 207
reduces the total deposition time further, as each head 202, 204,
206 may be configured to deposit its solution 203, 205, 207 on
different regions of the substrate 201. In some cases, the
deposition tool may even comprise a separate first 202, second 204
and/or third 206 deposition head for each quantum dot region 208,
thus allowing completion of the process in a matter of seconds.
This may be used to conveniently form an array of discreet
droplets/quantum dot regions. Furthermore, two or more of each type
of deposition head 202, 204, 206 may be configured to share a
common cartridge (instead of having their own individual
cartridges) to reduce the cost and complexity of the deposition
tool.
[0090] The deposition tool may utilise a number of different
dispensing technologies in order to deposit the various solutions
203, 205, 207. Suitable examples include inkjet (continuous or drop
on demand), electrospray, aerosol jet (e.g. Optomec jet 5x) and
valve-jet technologies.
[0091] In order to demonstrate the present method, a sample was
prepared by inkjet printing a 10 mg/I quantum dot solution
comprising PbS quantum dots stabilised by oleic acid (primary
ligands) in toluene using a Fuji Dimatix.TM. 4800 materials
deposition system. This deposition tool allows 1 picolitre drops to
be deposited with a placement accuracy of .about.50 .mu.m. The
quantum dot solution was deposited onto a graphene coated silicon
substrate 301 to form a plurality of droplets using different
numbers of ejected drops to vary the thickness of the droplets. In
particular, two rows of six droplets 311 were formed with 11 drops
per droplet, two rows of six droplets 312 were formed with 6 drops
per droplet, and two rows of six droplets 313 were formed with 4
drops per droplet.
[0092] FIGS. 3a-3c are optical micrographs of the substrate after
depositing the 11, 6 and 4-drop droplets, respectively. After
formation of the droplets of quantum dot solution, a
ligand-exchange solution comprising 2% ethanedithiol (secondary
ligands) in acetonitrile was deposited onto one row of each size of
droplet (rows 1, 3 and 6 in the micrographs) to replace the oleic
acid with ethanedithiol.
[0093] FIG. 3d is an optical micrograph of the substrate after
deposition of the ligand-exchange solution. After ligand
replacement, a first rinse solution comprising acetonitrile was
deposited onto the droplets to remove the oleic acid and any excess
ethanedithiol.
[0094] FIG. 3e is an optical micrograph of the substrate of FIG. 3d
following deposition of the first rinse solution. A second rinse
solution comprising toluene was then deposited onto the droplets to
help ensure removal of the oleic acid and any excess
ethanedithiol.
[0095] FIG. 3f is an optical micrograph of the substrate of FIG. 3e
following deposition of the second rinse solution.
[0096] FIGS. 4a-4c show optical micrographs of the substrate of
FIG. 3f taken at low, medium and high levels of magnification,
respectively. The three quantum dot regions 408 (droplets) visible
in FIG. 4c have been subjected to the ligand replacement process.
An atomic force microscope (AFM) was then used to determine the
thickness of the quantum dot regions 408 for comparison of regions
which were exposed to the ligand-exchange solution with those which
were not exposed to the ligand-exchange solution.
[0097] FIGS. 5a and 5b show an AFM image and corresponding height
profile for a quantum dot region 508b with no ligand replacement
and a quantum dot region 508a after ligand replacement,
respectively. The quantum dot regions 508a,b can be seen at the
right hand side of each image and were found to have a similar
thickness of 10-20 nm. Conductive AFM measurements were then
obtained for the quantum dot regions 508a,b of FIGS. 5a and 5b
using an Asylum MP3D in contact mode with a set point voltage of
0.2V, a silicon ASYELEC-01 Ir relex/tip coated conductive
cantilever and an applied tip voltage of 6V.
[0098] FIGS. 6a and 6b show a conductive AFM image for the quantum
dot region 608b with no ligand replacement and the quantum dot
region 608a after ligand replacement, respectively. In both cases,
there was found to be measured current flow through the underlying
graphene layer 614 (to the left of the quantum dot region 608a,b in
both images) of approximately 10 nA, but only the ligand-replaced
region 608a showed any conductivity through the quantum dots within
the region 608a.
[0099] FIG. 7 illustrates schematically one example of an apparatus
715 which can be formed using the method described herein. The
apparatus 715 comprises a plurality of quantum dots 716 (although
only one is shown here for ease of visualisation) on top of a layer
of electrically conductive material 714, the quantum dots
configured such that electromagnetic radiation 717 incident thereon
causes excitation of electrons 718 within the quantum dots 716
resulting in the generation of photo-excited charge 718. The
incident electromagnetic radiation 717 may comprise one or more of
radio waves, microwaves, infrared, visible light, ultraviolet,
x-rays, gamma rays and thermal radiation depending on the
dimensions of the quantum dots. In addition, the layer of
electrically conductive material 714 may be the supporting
substrate onto which the quantum dots 716 are deposited, or it may
be a separate charge collection layer on top of a supporting
substrate. The supporting substrate may comprise one or more of Si,
GaAs, BN, SiO.sub.2, LiF, Al.sub.2O.sub.3, HfO.sub.2 diamond like
carbon and graphene; and the electrically conductive material 714
may comprise one or more of carbon, graphene, reduced graphene
oxide, carbon nanotubes, a metal and a semiconductor.
[0100] The quantum dots 716 comprise the secondary ligands 719
attached thereto. Since the secondary ligands 719 have a relatively
short chain length (e.g. .ltoreq.1 nm) the photo-excited charge
718, generated from the quantum dots 716 by the incident
electromagnetic radiation 717, is transferred to the underlying
layer 714 to leave photo-excited charge 720 of opposite polarity on
the quantum dots 716. The photo-excited charge 720 of opposite
polarity generates an electric field causing a change in an
electrical property (e.g. conductivity, conductance, resistivity or
resistance) of the underlying layer 714 which can be used to detect
one or more of the presence and magnitude of the incident
electromagnetic radiation 717.
[0101] In order to enable the electrical property of the underlying
layer 714 to be determined, the apparatus 715 also comprises source
721 and drain 722 electrodes configured to cause a flow of charge
from the source electrode 721 through the layer of electrically
conductive material 714 to the drain electrode 722. The change in
electrical property may be determined using a voltmeter or ammeter
723, for example.
[0102] In this example, the quantum dots 716 are configured such
that the generated photo-excited charge 718 comprises the electrons
excited by the incident electromagnetic radiation 717, and the
photo-excited charge 720 of opposite polarity comprises
electron-holes left by the excited electrons. Furthermore, the
secondary ligands 719 are configured such that the transfer of
photo-excited charge 718 comprises tunnelling of the electrons
excited by the incident electromagnetic radiation 717 through the
secondary ligands 719 to the layer of electrically conductive
material 714.
[0103] In other examples, quantum dots 716 may be configured such
that the photo-excited charge 720 of opposite polarity comprises
the electrons excited by the incident electromagnetic radiation
717, and the generated photo-excited charge 718 comprises the
electron-holes left by the excited electrons. In this scenario, the
secondary ligands 719 may be configured such that the transfer of
photo-excited charge 718 comprises tunnelling of the electron-holes
through the secondary ligands 719 to the layer of electrically
conductive material 714.
[0104] The charge transfer mechanism is not limited solely to
electron/hole tunnelling, however. In some examples, electron
thermally-activated electron/hole hopping may be used. Transfer of
the photo-excited charge 718 is not only governed by the secondary
ligands 719, but also by the band structure between the quantum
dots 716 and the underlying layer of material 714. This may be
affected by doping or bias voltages, which could be used to tune
the optoelectrical response of the apparatus.
[0105] As mentioned above, the quantum dot solution used to
transport the quantum dots to the supporting substrate comprises a
plurality of quantum dots having primary ligands attached thereto
to stabilise the quantum dots in solution. In some examples, the
quantum dot solution may further comprise an electrically
conductive material such that the quantum dots and electrically
conductive material are arranged within the quantum dot solution to
form a composite material. In this scenario, the electrically
conductive material may facilitate charge transfer between the
quantum dots and from the quantum dots to the underlying layer of
electrically conductive material.
[0106] Also, although the absorption wavelength of quantum dots can
be tuned by controlling their size, their selectivity/sensitivity
can be further controlled by depositing a functional material onto
the quantum dots to functionalise them for a particular
application. For example, a photosensitive semiconducting material
such as a conjugated polymer, dye or metal nanoparticles may be
used to respond to a particular wavelength of light. The
selectivity/sensitivity of the quantum dots may also be modified by
functionalising them with biomaterials such as antibodies,
antigens, proteins or DNA fragments. In some cases, these
biomaterials may be used to interact specifically with other
biological molecules to alter the selectivity/sensitivity to
light.
[0107] For most applications, the discrete quantum dot regions on
the substrate will be configured to form an optically sensitive
surface. In order to improve the chances of detecting incident
electromagnetic radiation, the quantum dot solution should be
deposited to maximise the areal coverage of the quantum dots. In
this respect, the quantum dot solution may be deposited such that
the two or more discrete droplets (which will typically have a
diameter of less than or equal to 10 .mu.m, 25 .mu.m, 50 .mu.m or
100 .mu.m) are deposited to form a one, two or three dimensional
array and are spaced apart by no more than 5 .mu.m, 10 .mu.m or 15
.mu.m. This can be achieved by controlling the position, or drop
firing frequency of the deposition head or underlying substrate
together with the volume of each drop leaving the deposition head.
A number of different deposition techniques can be used to form
such an array. Suitable examples include conventional inkjet
printing (to produce a drop size of .ltoreq.1 .mu.l), inkjet
deposition (e.g. using drop-on-demand, thermal or piezo heads to
produce a drop size of .ltoreq.1 .mu.l), continuous inkjet using
controlled droplet breakup to select printing or non-printing (e.g.
via thermal pulse, piezo pulse or electrostatic force to produce a
drop size of .ltoreq.1 .mu.l), electrospray (aka electrostatic drop
generation or electrohydrodynamic to produce a drop size of
.ltoreq.1 pl), and valve-jet (to produce a drop size of .ltoreq.1
ml).
[0108] FIG. 8 illustrates schematically an apparatus 815 according
to another embodiment of the present disclosure. The apparatus 815
may be one or more of an electronic device, a portable electronic
device, a portable telecommunications device, a mobile phone, a
personal digital assistant, a tablet, a phablet, a desktop
computer, a laptop computer, a server, a smartphone, a smartwatch,
smart eyewear, a sensor, a camera, an image sensor, a
photodetector, a phototransistor, and a module for one or more of
the same. In the example shown, the apparatus 815 comprises two or
more discrete quantum dot regions 808 and an electrically
conductive material 814 on the surface of a supporting substrate
801, first 821 and second 822 electrodes, a power source 824, an
ammeter 823, a voltmeter 825, a processor 826 and a storage medium
827, which are electrically connected to one another by a data bus
828.
[0109] The power source 824 is configured to apply a voltage
between the source and drain electrodes, the voltmeter 825 is
configured to measure the applied voltage, and the ammeter 823 is
configured to measure the resulting current flowing through the
electrically conductive material.
[0110] The processor 826 is configured for general operation of the
apparatus 815 by providing signalling to, and receiving signalling
from, the other components to manage their operation. In addition,
the processor 826 is configured to receive the voltage and current
measurements from the voltmeter 825 and ammeter 823, respectively,
determine an electrical property of the electrically conductive
material using the voltage and current measurements, and determine
one or more of the presence and magnitude of incident
electromagnetic radiation based on the determined electrical
property.
[0111] The storage medium 827 is configured to store computer code
configured to perform, control or enable operation of the apparatus
815. The storage medium 827 may also be configured to store
settings for the other components. The processor 826 may access the
storage medium 827 to retrieve the component settings in order to
manage the operation of the other components. The storage medium
827 may also be configured to store calibration data (e.g.
predetermined measurements of intensity levels of incident
electromagnetic radiation versus a corresponding electrical
property) for use by the processor 826 in determining one or more
of the presence and magnitude of the incident electromagnetic
radiation.
[0112] The processor 826 may be a microprocessor, including an
Application Specific Integrated Circuit (ASIC). The storage medium
827 may be a temporary storage medium such as a volatile random
access memory. On the other hand, the storage medium 827 may be a
permanent storage medium such as a hard disk drive, a flash memory,
or a non-volatile random access memory.
[0113] FIG. 9 illustrates schematically the main steps 929-930 of
the method described herein. As shown, the method generally
comprises: depositing a quantum dot solution onto a supporting
substrate in a drop-wise manner to form two or more discrete
droplets on the surface of the substrate 929; and depositing a
ligand-exchange solution onto the discrete droplets in a drop-wise
manner to cause replacement of the primary ligands with
shorter-chain secondary ligands 930.
[0114] FIG. 10 illustrates schematically a computer/processor
readable medium 1031 providing a computer program according to one
embodiment. In this example, the computer/processor readable medium
1031 is a disc such as a digital versatile disc (DVD) or a compact
disc (CD). In other embodiments, the computer/processor readable
medium 1031 may be any medium that has been programmed in such a
way as to carry out an inventive function. The computer/processor
readable medium 1031 may be a removable memory device such as a
memory stick or memory card (SD, mini SD, micro SD or nano SD).
[0115] The computer program may comprise computer code configured
to perform, control or enable one or more of the method steps
929-930 of FIG. 9, e.g. by controlling the deposition tool used to
deposit the quantum dot and ligand-exchange solutions. Additionally
or alternatively, the computer program may be configured to
measure/determine the difference in the electrical property (e.g.
conductivity, conductance, resistance, resistivity, etc) of the
electrically conductive material 714, 814 of the apparatus 715, 815
of FIG. 7 or 8, and determine one or more of the presence and
magnitude of the incident electromagnetic radiation 717.
[0116] Other embodiments depicted in the figures have been provided
with reference numerals that correspond to similar features of
earlier described embodiments. For example, feature number 1 can
also correspond to numbers 101, 201, 301 etc. These numbered
features may appear in the figures but may not have been directly
referred to within the description of these particular embodiments.
These have still been provided in the figures to aid understanding
of the further embodiments, particularly in relation to the
features of similar earlier described embodiments.
[0117] It will be appreciated to the skilled reader that any
mentioned apparatus/device and/or other features of particular
mentioned apparatus/device may be provided by apparatus arranged
such that they become configured to carry out the desired
operations only when enabled, e.g. switched on, or the like. In
such cases, they may not necessarily have the appropriate software
loaded into the active memory in the non-enabled (e.g. switched off
state) and only load the appropriate software in the enabled (e.g.
on state). The apparatus may comprise hardware circuitry and/or
firmware. The apparatus may comprise software loaded onto memory.
Such software/computer programs may be recorded on the same
memory/processor/functional units and/or on one or more
memories/processors/functional units.
[0118] In some embodiments, a particular mentioned apparatus/device
may be pre-programmed with the appropriate software to carry out
desired operations, and wherein the appropriate software can be
enabled for use by a user downloading a "key", for example, to
unlock/enable the software and its associated functionality.
Advantages associated with such embodiments can include a reduced
requirement to download data when further functionality is required
for a device, and this can be useful in examples where a device is
perceived to have sufficient capacity to store such pre-programmed
software for functionality that may not be enabled by a user.
[0119] It will be appreciated that any mentioned
apparatus/circuitry/elements/processor may have other functions in
addition to the mentioned functions, and that these functions may
be performed by the same apparatus/circuitry/elements/processor.
One or more disclosed aspects may encompass the electronic
distribution of associated computer programs and computer programs
(which may be source/transport encoded) recorded on an appropriate
carrier (e.g. memory, signal).
[0120] It will be appreciated that any "computer" described herein
can comprise a collection of one or more individual
processors/processing elements that may or may not be located on
the same circuit board, or the same region/position of a circuit
board or even the same device. In some embodiments one or more of
any mentioned processors may be distributed over a plurality of
devices. The same or different processor/processing elements may
perform one or more functions described herein.
[0121] It will be appreciated that the term "signalling" may refer
to one or more signals transmitted as a series of transmitted
and/or received signals. The series of signals may comprise one,
two, three, four or even more individual signal components or
distinct signals to make up said signalling. Some or all of these
individual signals may be transmitted/received simultaneously, in
sequence, and/or such that they temporally overlap one another.
[0122] With reference to any discussion of any mentioned computer
and/or processor and memory (e.g. including ROM, CD-ROM etc), these
may comprise a computer processor, Application Specific Integrated
Circuit (ASIC), field-programmable gate array (FPGA), and/or other
hardware components that have been programmed in such a way to
carry out the inventive function.
[0123] The applicant hereby discloses in isolation each individual
feature described herein and any combination of two or more such
features, to the extent that such features or combinations are
capable of being carried out based on the present specification as
a whole, in the light of the common general knowledge of a person
skilled in the art, irrespective of whether such features or
combinations of features solve any problems disclosed herein, and
without limitation to the scope of the claims. The applicant
indicates that the disclosed aspects/embodiments may consist of any
such individual feature or combination of features. In view of the
foregoing description it will be evident to a person skilled in the
art that various modifications may be made within the scope of the
disclosure.
[0124] While there have been shown and described and pointed out
fundamental novel features as applied to different embodiments
thereof, it will be understood that various omissions and
substitutions and changes in the form and details of the devices
and methods described may be made by those skilled in the art
without departing from the spirit of the invention. For example, it
is expressly intended that all combinations of those elements
and/or method steps which perform substantially the same function
in substantially the same way to achieve the same results are
within the scope of the invention. Moreover, it should be
recognized that structures and/or elements and/or method steps
shown and/or described in connection with any disclosed form or
embodiment may be incorporated in any other disclosed or described
or suggested form or embodiment as a general matter of design
choice. Furthermore, in the claims means-plus-function clauses are
intended to cover the structures described herein as performing the
recited function and not only structural equivalents, but also
equivalent structures. Thus although a nail and a screw may not be
structural equivalents in that a nail employs a cylindrical surface
to secure wooden parts together, whereas a screw employs a helical
surface, in the environment of fastening wooden parts, a nail and a
screw may be equivalent structures.
* * * * *